1. Field of the Invention
This invention relates to an optical recording medium having a recording portion for signal recording, formed on one of the major surfaces of a support and a light transmitting layer formed on the recording portion. The light is illuminated from the side of the light transmitting layer to record and/or reproduce information signals.
2. Description of the Related Art
An optical recording medium on one surface of which recording and/or reproduction for four hours is possible in accordance with the NTSC (National Television System Committee) has so far been proposed. In this manner, the optical recording medium has the function as a new recording medium which takes the place of a video tape cassette used in the current VTR (video tape recorder).
On the other hand, a demand is raised to make the shape or the size of an optical recording medium equivalent to those of a CD (Compact Disc) to render the recording medium more friendly to the user accustomed to the ease in handling and usability of the CD. On the other hand, a demand is raised to constitute the optical recording medium as a disc-shaped recording medium similar to a CD to exploit random-accessibility and fast accessing proper to the disc configuration to provide a recording medium which is small-sized and easy to operate, capable of instantaneous recording and/or reproduction and which has diversified functions such as tricky play or prompt editing.
The optical recording medium is required to exhibit diversified capabilities and properties, for use as the next-generation recording medium, and hence is in need of e.g., a recording capacity of not less than 8 GB.
However, the recording capacity of a conventional optical recording medium is not larger than 8 GB. As a conventional optical recording medium, a DVD (Digital Versatile Disc) has already been proposed. In DVD, the recording wavelength λ is 0.65 μm, the numerical aperture NA is 0.6 and the recording capacity is 4.7 GB.
If, with equivalent signal formats, such as the ECC (error correction code) or the modulation system, to those of the DVD, the recording capacity of an optical recording medium is to be not less than 8 GB, the relationship:4.7×(0.65/0.60×λ)2≧8 needs to be met. By solving this formula, NA/λ≧1.20. Therefore, if, in an optical recording medium, the recording capacity is to be not less than 8 GB, the numerical aperture NA needs to be of a larger value, or the recording wavelength λ needs to be smaller.
If, in an optical recording medium, the numerical aperture NA is of a larger value, the allowance of the angle with which the disc surface deviates from the optical axis of the optical pickup (tilt angle) is diminished. Thus, in above-described optical recording medium in which the aberration due to the thickness of the disc surface tends to be affected by the tilt angle, the light transmitting layer through which is transmitted the illuminating light needs to be reduced to achieve stabilized signal recording and/or reproduction. In the optical recording medium, thickness variations in the light transmitting layer need to be smaller than a pre-set value, for the same reason.
The optical recording medium has a merit that, if the light transmitting layer is reduced in thickness, a higher recording density is achieved. It has, however, a drawback that it tends to be affected significantly by scratches or dust and dirt on the disc surface to render signal recording and/or reproduction difficult. That is, if, in an optical recording medium, recording and/or reproduction is to be performed using an optical system of high recording density employing an objective lens of high numerical aperture, it is necessary to reduce the working distance, that is the distance between the objective lens and the recording and/or reproducing surface of the disc, in comparison with that of a conventional optical recording medium. At this time, the optical recording medium tends to be damaged due to the increased probability of collision between the disc surface and the objective lens. In such case, the amount of dust and dirt affixed to the disc surface of the optical recording medium is increased due to electrification of the disc surface. The result is the increased rate of the recording and/or reproducing errors due to scratches or the dust and dirt on the disc surface.